Fiber-reinforced plastics have excellent specific strength and specific stiffness, and thus their applications have been widely expanded to aircraft structural members, wind mill blades, and automobile outer panels, as well as computer applications including IC trays, laptop computer housings, and the like, and the demand is increasing year by year. In particular, carbon-fiber-reinforced composite materials are light in weight and have excellent strength and stiffness, and thus have been widely used in the aircraft industry, as typified by commercial aircrafts. In recent years, they are also used for large-sized structural members, such as main wings and bodies.
For such a structural member, a fiber-reinforced plastic laminate composed of prepreg sheets that have been laid up and then cured is often used. In a fiber-reinforced plastic laminate, carbon fibers are unidirectionally aligned, and thus the fiber volume content can be increased, whereby the high elastic modulus and strength of carbon fibers can be maximumly utilized. In addition, when the prepreg is impregnated with a high-performance resin while reducing variation in areal weight, the obtained fiber-reinforced plastic laminate has stable quality. However, a fiber-reinforced plastic laminate has a drawback in that delamination tends to occur between the prepreg layers upon the impact of a foreign substance from out of the plane. The occurrence of delamination leads to a decrease in the compression strength of the structural member, and thus compressive strength after impact (CAI) has long since been a design constraint issue in the designing of aircrafts, and the enhancement of CAI has been the most important issue in the material designing of fiber-reinforced plastics. Against such a background, Patent Document 1 proposes a prepreg on the surface of which a thermoplastic resin formed into fine particles is localized. Sheets of such a prepreg which are laid up and formed into a fiber-reinforced plastic laminate can have a highly toughened thermoplastic resin localized between the layers, have a reduced area of delamination upon the application of out-of-plane impact, and have an enhanced CAI. Currently, such an “increased-interlayer-toughness” prepreg has been mainly applied to fiber-reinforced plastics used for primary structural members of aircrafts.
It is known that, of the steps of producing a structural member, the shaping step, in which prepreg is made to conform to a three dimensional shape of interest and formed into a preform before the forming/curing step using an autoclave or the like, is an important step that influences the success or failure in material quality. When prepreg layers are shaped layer by layer, a high-quality preform can be obtained, but such a process is costly and also takes a long period of time. Then, in order to enhance the production efficiency, a shaping method called hot-forming, in which prepreg sheets are previously laid up in planar form into a prepreg laminate at high speed using an automatic machine, and then the prepreg laminate is shaped into a three dimensional shape while heat is applied thereto, is used. Patent Document 2 discloses a shaping method in which a prepreg laminate is disposed between a mandrel and an expandable bladder, and the bladder is expanded, thereby pressing the laminate against the mandrel while bending the same.